Market Sizing Examples: Four Worked Solutions for Interviews
The market sizing case interview is not about getting the right number. It is about showing structured thinking, explicit assumptions, and numerical fluency under time pressure. This guide walks through four fully worked market sizing examples using both top-down and bottom-up approaches, so you can see exactly how strong candidates talk through the calculation.
What is a Market Sizing Question?
A market sizing or guesstimate question asks you to estimate the size of a market, a volume, or a count with limited data. Typical prompts: "How many coffee shops are in Paris?" or "How many tennis balls are sold in the US each year?"
Interviewers score you on structure, assumptions, math, and sanity-checking. A candidate within 50% of reality with a clean approach scores higher than one who guesses correctly without structure.
Top-Down Versus Bottom-Up
Every market sizing question can be tackled two ways. Strong candidates pick one, solve it cleanly, and cross-check with the other when time allows.
| Approach | Starts from | Best for |
|---|---|---|
| Top-down | Population or macro total | Consumer markets, frequency-based |
| Bottom-up | Units, outlets, or transactions | Infrastructure, supply-side questions |
Insider Tip: State your approach explicitly. "I will take a top-down approach, starting from the population of France." This single sentence earns structure points immediately.
Essential Anchor Numbers
Memorise a short list of anchors. You will reuse them in 80% of market sizing questions.
| Anchor | France | Germany | UK | US |
|---|---|---|---|---|
| Population | 68M | 84M | 67M | 330M |
| Households | 30M | 41M | 28M | 130M |
| GDP | 2.8T EUR | 4.1T EUR | 3.1T GBP | 28T USD |
| Average salary | 38K EUR | 45K EUR | 35K GBP | 55K USD |
You should also know rough life expectancy at 80 years, average household size between two and three, and urbanisation rates in developed markets around 80%.
Worked Example 1: Coffee Shops in Paris
Approach: bottom-up from demand.
Paris has around 2.2 million residents plus roughly 30 million annual tourists, which translates to about 2.5 million daily equivalents. Assume 50% of adults buy coffee from a shop on any given day, giving around 1.2 million daily coffee transactions.
A typical independent coffee shop sells around 200 coffees per day. Dividing 1.2 million daily transactions by 200 gives roughly 6,000 outlet-equivalents. Many of those transactions happen at chains and bakeries, so pure coffee shops are probably closer to half that, around 3,000.
Cross-check top-down: 2.2 million residents times 0.5 shops per 1,000 inhabitants gives 1,100, which is low because it ignores tourism. The bottom-up figure of around 3,000 is more defensible.
Worked Example 2: Tennis Balls Sold in the US Each Year
Approach: top-down by segment.
Start with three demand segments: recreational players, tournament and club play, and professional or training use.
- Recreational: 20 million occasional players times 6 balls per year equals 120 million
- Club and tournament: 5 million active players times 30 balls per year equals 150 million
- Professional and training: negligible at the total market level, maybe 10 million
Add cross-check demand from non-tennis uses such as dog toys and chair feet, which adds roughly 30 to 50 million. Total sits near 300 to 330 million balls per year.
This example shows the value of segmenting before multiplying. A single average would badly under or over-shoot.
Worked Example 3: New EV Charging Stations Needed in the UK by 2030
Approach: bottom-up from the car parc.
The UK has around 33 million passenger cars on the road. Assume 50% are electric by 2030, giving around 16 million EVs.
Industry rules of thumb suggest one public fast charger per 10 to 20 EVs, plus home chargers for around 60% of owners. The public charger need is 16 million divided by 15, or roughly 1 million public chargers. The UK currently has around 60,000 public chargers, so the gap is close to 900,000 new units by 2030.
Cross-check: that implies adding roughly 150,000 chargers per year, which is plausible given current installation rates of around 20,000 per year trending up sharply.
Worked Example 4: Annual Revenue of the French Fitness Market
Approach: top-down from population.
France has 68 million people. Assume 10% are active gym members, giving 6.8 million members. Average membership is around 35 euros per month, or 420 euros per year. Core gym revenue is 6.8 million times 420, or roughly 2.9 billion euros.
Add adjacent revenue from boutique studios, personal training, and corporate wellness at roughly 40% of core. Total market is around 4 billion euros.
Cross-check bottom-up: France has roughly 4,500 gyms with average revenue of 800,000 euros each, giving 3.6 billion. Plus boutique and PT revenue, the convergence around 3.5 to 4 billion is robust.
Common Mistakes to Avoid
- Jumping into math without stating the approach
- Using a single average rather than segmenting
- Forgetting to sanity check with a different method
- Refusing to round or chasing false precision
- Ignoring the range and caveats at the end
Strong candidates finish with a range, such as "I estimate 3.5 to 4 billion euros, with the main sensitivity on membership rate and average price." This shows numerical humility and business judgment.
Pro Tip: End every sizing with one sentence on sensitivity. "If the membership rate is 8% rather than 10%, the market is closer to 3.2 billion." This almost always earns an extra point on structure.
Practice and Next Steps
Market sizing rewards reps. Aim for at least 20 varied questions across consumer, industrial, and infrastructure categories before your interview. Pair this with our case interview frameworks guide to tie sizing into broader cases.
FAQ
Top-down or bottom-up: which is better?
Neither is universally better. Top-down works for consumer markets tied to population. Bottom-up works for supply-side or infrastructure questions. The best candidates pick one, solve it, and cross-check with the other.
How much time should a market sizing take?
Aim for three to five minutes. Anything longer suggests the structure is unclear.
Do I need to memorise country populations?
Yes for your target markets and major economies. These anchors appear in the majority of sizing prompts.
What if my number is off by a factor of two?
That is usually fine if your approach was clean and your assumptions were stated. Interviewers care about reasoning quality more than numerical precision.
Should I always cross-check with a second method?
If time permits, yes. Even a rough cross-check shows rigour and catches order-of-magnitude errors.
Further Reading
- McKinsey Quarterly, Latest articles
- BCG, Publications
- Bain & Company, Bain Insights
- Harvard Business Review, Strategy topic archive
- The Economist, Business section
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